1. Field of the Invention
The present invention relates to an image forming apparatus in which an accessory unit is mounted to the main body for promoting the image forming operation.
2. Description of the Related Art
Image forming apparatuses, constituted by copiers, are able to copy both sides, of a single document onto both sides of a paper P. These have improved in reliability in recent years, and their cost has come down as a result, their use is increasing.
FIG. 7 shows a diagrammatic cross-sectional view of a copier provided with a paper feed unit or an Automatic Duplex Device 1 (hereinbelow abbreviated as ADD) for two-sided copying, and a Reversible Automatic Document Feeder (hereinbelow abbreviated as RADF) of the both-sides paper-feed type. FIG. 8 shows a time chart of the copying process.
Both-sides copying operation is commenced by pressing a both-sides copy mode key (not shown), and a print key (not shown). First, an ADD main motor (not shown) is rotated, causing changeover gate 9 to change over to the direction of stacker 3 in which sheets of paper P are stored. At the same time, paper side guide 5 and paper rear end guide 7 of stacker 3 are moved by a driving means (described later) into positions corresponding to the selected paper size.
Next, paper P extracted by paper feed roller 13 from paper tray 11 is fed to photosensitive body 21 of image forming unit 19 by means of feed roll 15 and aligning roll 17. Copying of the first side of paper P is then performed as follows. Toner is transferred from photosensitive body 21 by means of transfer charger 23. The paper is then separated from photosensitive body 21 by the action of separating charger 25 (process A), and fed to fixing unit 29 by conveyor belt 27, thus completing the copying of the first side. After this, paper P is discharged by discharge roll 31 and fed into stack feed path 33 of ADD 1 by means of changeover gate 9 (process B). After completion of process A, the document is turned over by the RADF, to prepare for the copying of the second side of the paper.
Paper P in stack feed path 33 is fed to stacker 3 by conveyor rolls 35, 37, 39, 41, and 43, and is stacked in stacker 3 with the copied side upwards (process C). The above processes A to C are repeated until the designated number of copied sheets of paper are stacked in stacker 3.
Next, pickup roll 45 is lowered by means of the pickup solenoid, so that pickup roll 45 feeds the first paper P in stacker 3 to separating roll 47. Changeover gate 9 also changes over towards the side of discharge roll 49.
The leading end of paper P that is, thus, fed from stacker 3 reaches aligning roller 51, which is stationary, and is aligned by pressing against this roller 51 (process D). When aligning roller 51 starts to rotate, paper P is fed to aligning roller 17 of image forming unit 19, where it is aligned, and transfer is effected to the second side of paper P. Paper P is then separated from photosensitive body 21 (process E). After paper P has been separated from photosensitive body 21, fixing is performed on the second side of paper P. Paper P is then passed from discharge roller 31 through the top of changeover gate 9 and then through discharge roller 49, whence paper P is discharged to paper receiving tray 53 (process F). After completion of the above process E, the RADF feeds the next document D.
Thus, in the both-sides copying mode, as shown in FIG. 8, the time required to copy both sides (front and back) of a single-sheet document is 8.5 seconds and a time of 8.5/2=4.25 seconds is required for copying of one side. In contrast, in single-side copying mode, in which only one side of paper P is copied, as shown by the time chart of FIG. 9, five copies are made in a period of 6.5 seconds. In this case, the time for copying one side is only 6.5/5=1.3 seconds.
Thus, when the both-sides copying mode is employed, the copying time per side is longer than the case of single-side copying mode. Furthermore, the operating time of the image forming unit 19 becomes longer. This causes the following problems.
(1) Since the developer is stirred for a long time in the developing unit 55 deterioration of its developing characteristics due to oxidation of the developer is promoted. Furthermore, this protracted stirring increases the amount of splashing of developer to outside the developing unit 55, resulting in increased soiling of the area around this unit 55.
(2) At the periphery of photosensitive body 21, cleaning blade 59 of cleaning unit 57, which is always in contact with photosensitive body 21 while this body rotates during copying, is subjected to friction over a long period of time. This promotes wear of cleaning blade 59, and dulls its edge, adversely affecting its cleaning effect.
(3) Due to the friction between photosensitive body 21 and cleaning blade 59, local wear of photosensitive body 21 is increased, causing damage to photosensitive body 21 and deterioration in its performance.
(4) Regarding the fixing unit 29, wear of the heat roll 56 and cleaning felt 60, that cleans away dirt, is increased by protracted operation. This adversely affects the fixing function, and shortens the life, of these two components.